"Are the things good?"
Translation:Jsou ty věci dobré?
I don't think there are any comprehensive rules written anywhere.
In general, if the definite article "the" is mandated by grammar in English, i.e. you have no option of leaving it out, such as "the first", "the last", or "in the world", or "the definite article" at the (!) beginning of this sentence, then there is no demonstrative in Czech.
But if the definite article in English actually works as a distinction from a possible indefiniteness (e.g. "I see a cat" vs. "I see the cat"), then Czech will usually use a demonstrative: "Vidím kočku" vs. "Vidím tu kočku".
If you are standing in front of (those or) things, you might leave the "ty" of, if there are no things present, you have to specify, if you are talking of all things of the universe, or some specific ones! That's the question i asking myself to help me out of this dilemma. : )